Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752869AbZLaBjk (ORCPT ); Wed, 30 Dec 2009 20:39:40 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752655AbZLaBjj (ORCPT ); Wed, 30 Dec 2009 20:39:39 -0500 Received: from mga14.intel.com ([143.182.124.37]:60267 "EHLO mga14.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752383AbZLaBjj (ORCPT ); Wed, 30 Dec 2009 20:39:39 -0500 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="4.47,316,1257148800"; d="scan'208";a="227984617" Date: Thu, 31 Dec 2009 09:39:36 +0800 From: Wu Fengguang To: Andi Kleen Cc: Andrew Morton , Quentin Barnes , "linux-kernel@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , Nick Piggin , Steven Whitehouse Subject: Re: [RFC][PATCH 2/2] readahead: avoid page-by-page reads on POSIX_FADV_RANDOM Message-ID: <20091231013935.GA6570@localhost> References: <20091225000717.GA26949@yahoo-inc.com> <87aax18xms.fsf@basil.nowhere.org> <20091230051540.GA16308@localhost> <20091230052402.GB26364@localhost> <873a2s8hmp.fsf@basil.nowhere.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <873a2s8hmp.fsf@basil.nowhere.org> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1706 Lines: 42 On Thu, Dec 31, 2009 at 02:02:38AM +0800, Andi Kleen wrote: > Wu Fengguang writes: > > * the ra fields can be accessed concurrently in a racy way. > > --- linux.orig/mm/fadvise.c 2009-12-30 13:02:03.000000000 +0800 > > +++ linux/mm/fadvise.c 2009-12-30 13:23:05.000000000 +0800 > > @@ -77,12 +77,14 @@ SYSCALL_DEFINE(fadvise64_64)(int fd, lof > > switch (advice) { > > case POSIX_FADV_NORMAL: > > file->f_ra.ra_pages = bdi->ra_pages; > > + file->f_ra.flags &= ~RA_FLAG_RANDOM; > > break; > > case POSIX_FADV_RANDOM: > > - file->f_ra.ra_pages = 0; > > + file->f_ra.flags |= RA_FLAG_RANDOM; > > What prevents this from racing with a parallel readahead > state modification, losing the bits? Oh I pretended that the problem don't exist.. To be serious, the race only exist inside a mutithread application, where one single fd is shared between two threads, one is doing fadvise, another doing readahead. A sane application won't do fadvise(POSIX_FADV_RANDOM) while active reads are going one concurrently: this leads to indeterminate behavior by itself -- from which request the random hint takes effect? fadvise() shall always be in the same streamline with all reads. In real workloads, 1% applications may do POSIX_FADV_RANDOM, among which 1% applications may be broken. And if the race does happen, the impact is very small. So I choose to just ignore the race and use non-atomic operations.. Thanks, Fengguang -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/